Update sent timestamp when write irrecoverably fails.

We have an alert that fires when prometheus_remote_storage_highest_timestamp_in_seconds - prometheus_remote_storage_queue_highest_sent_timestamp_seconds
becomes too high. But we have an agent that fires this when the remote "rate-limits" the user.

This is because prometheus_remote_storage_queue_highest_sent_timestamp_seconds doesn't get updated
when the remote sends a 429.

I think we should update the metrics, and the change I made makes sense. Because if the requests fails
because of connectivity issues, etc. we will never exit the `sendWriteRequestWithBackoff` function. It only
exits the function when there is a non-recoverable error, like a bad status code, and in that case, I think
the metric needs to be updated.

Signed-off-by: Goutham Veeramachaneni <gouthamve@gmail.com>

storage/remote/queue_manager.go

@@ -1311,12 +1311,11 @@ func (s *shards) sendSamplesWithBackoff(ctx context.Context, samples []prompb.Ti
 	}
 
 	err = sendWriteRequestWithBackoff(ctx, s.qm.cfg, s.qm.logger, attemptStore, onRetry)
-	if err != nil {
-		return err
-	}
+
 	s.qm.metrics.sentBytesTotal.Add(float64(reqSize))
 	s.qm.metrics.highestSentTimestamp.Set(float64(highest / 1000))
-	return nil
+
+	return err
 }
 
 func sendWriteRequestWithBackoff(ctx context.Context, cfg config.QueueConfig, l log.Logger, attempt func(int) error, onRetry func()) error {